Neoteny, or the retention of juvenile traits, may have played a part in our transition from more ape-like characteristics to our current form. I learned just the other day that full-term chimpanzee and human fetus skulls are strikingly similar.

The major differences in skull structure only become apparent as individuals get closer to sexual maturity.

Human skulls are also similar to other primate children.

Humans have a much longer period of childhood than modern apes do, so this might explain the retention of such traits over a longer period. However, the Russian Silver Fox experiment shows that gracility is associated with lower levels of adrenaline. The bonobo is often (and incorrectly) called the "Pigmy Chimpanzee" because of its more gracile build than its stockier cousin. As we all know, they are far more peaceful than chimps. So the transition to a more egalitarian society as apposed to the competition driven environment of our ancestors may have been a driver of neoteny.